The present invention relates to a copper-clad laminate including an insulative substrate having a copper foil laminated thereto, a circuit board for a printed wiring board incorporating the copper-clad laminate, and method for producing the circuit board.
For a high adhesion between the copper foil and resin and better rust prevention, the copper-clad laminate for use to produce a printed wiring board is normally subjected at the copper foil surface thereof to a surface roughening or a surface processing such as coating of zinc or the lie. For example, when a multilayer printed wiring board is produced using the copper-clad laminate, the copper foil surface is etched to form a conductive circuit, through-holes are formed as extended from the resin surface opposite to the copper foil to the conductive circuit by laser irradiation, and further a conductive substance is filled in each through-hole to form a viahole. A plurality of such circuit boards is stacked one on the other with an adhesive layer being laid between them, and they are hotxe2x80x94pressed together to produce a multilayer printed wiring board having an IVH structure. Such multilayer printed wiring board and a method for production thereof are disclosed in the Japanese Patent Application No. 7-139878 of the Application of the present invention.
To form the viaholes extending from the resin layer surface to the conductive circuit, the through-holes can be formed very precisely and in a short time by the use of an appropriate laser irradiation. The lasing conditions including a laser output, for forming such through-holes, depends upon the thickness of the resin layer and copper foil, but the laser output is limited to a predetermined range which will not damage the copper foil.
In case the through-hole is formed by the laser drilling, however, smear will remain on the inner wall surface of the through-hole. After through-holes are formed, they have to be desmeared by a chemical processing or plasma processing. The smearing residue on the entire bottom of the through-hole was examined and analyzed. The analysis result proved that a part of zinc applied to the matted or mat-finished surface of the copper foil for the purpose of improving the adhesion to the resin layer surface cannot completely be evaporated under the predetermined lasing conditions but it is melted to be integral with the resin fibers and adheres to the entire bottom of the through-holes.
Accordingly, the present invention has a main object to overcome the above-mentioned drawbacks of the prior art by providing a copper-clad laminate which can be produced without having to desmear a through-holes for viaholes.
The present invention also has an object to provide a circuit board for a printed wiring board, in which a copper-clad laminate producible without having to desmear through-holes for viaholes, and a method for production of the circuit board.
To attain the above objects, the inventors of the present invention had made many experiments and found that a metal layer whose melting point is lower than that of zinc and almost all of which are evaporated under the predetermined lasing conditions can be formed on a roughened side, that is, a mat-finished surface, of a copper foil. Based on the above findings, the inventors of the present invention provide the following inventions:
(1) A copper-clad laminate including an insulative substrate having laminated on one or either side thereof a copper foil whose one side is roughened,
the copper foil having formed on the roughened surface side thereof a metal layer whose melting point is lower than that of zinc.
(2) A circuit board for a printed wiring board, including an insulative substrate having a conductive circuit formed on one side thereof and viaholes formed in through-holes extending from the other side of the insulative substrate to the conductive circuit,
there being formed between the one side of the insulative substrate and the conductive circuit a metal layer whose melting point is lower than that of zinc.
In the above copper-clad laminate (1) and circuit board (2), the metal layer lower in melting point than zinc should preferably be formed from a solder containing tin as a main component such as a lead-tin solder, silver-tin solder, bismuth-tin solder or the like and having a thickness of 0.1 to 2.0 xcexcm.
(3) A method for producing a circuit board for a printed wiring board, including an insulative substrate having a conductive circuit formed on one side thereof and viaholes formed in through-holes extending from the other side of the insulative substrate to the conductive circuit and in which there is formed between the one side of the insulative substrate and the conductive circuit a metal layer whose melting point is lower than that of zinc, the method including the following steps (a) to (d):
(a) roughening one side of a copper foil and then forming, on the roughened surface, a metal layer whose melting point is lower than that of the roughened surface of the copper foil;
(b) laminating the copper foil at the side thereof on which the metal layer is formed to one side of the insulative substrate;
(c) etching the copper foil attached to the insulative substrate to form the conductive circuit; and
(d) forming through-holes extending from the other side of the insulative substrate to the conductive circuit by lasing and then forming viaholes in the through-hole.
(4) A method for producing a circuit board for a printed wiring board, including an insulative substrate having a conductive circuit formed on one side thereof and a viahole formed in through-holes extending from the other side of the insulative substrate to the conductive circuit and in which there is formed between the one side of the insulative substrate and the conductive circuit a metal layer whose melting point is lower than that of zinc, the method including the following steps (a) to (d):
(a) forming a low melting point metal layer on one side of the insulative substrate by the PVD or CVD method or electroless plating;
(b) making an electrolytic copper plating to the insulative substrate having the low melting point metal layer formed thereon to provide a conductive circuit having a copper plated layer formed on the low melting point metal layer;
(c) etching the conductive layer to the conductive circuit on one side of the insulative substrate; and
(d) forming through-holes extending from the other side of the insulative substrate to the conductive circuit by lasing and then forming filled-viaholes in the through-holes.
In the above methods (3) and (4), the viahole should preferably be formed by filing a conductive substance in the through-hole.
These objects and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the best modes for carrying out the present invention when taken in conjunction with the accompanying drawings.